It is becoming evident that epigenetic mechanisms, including DNA methylation, histone modification and non-coding RNAs, play significant roles in spermatogenesis. Despite recent progress, our comprehension of the dynamic epigenome in developing male germ cells remains largely to be defined. Thus, it is time to identify novel epigenetic regulators and discover how they work in spermatogenesis. The goal of this application is to test the hypothesis that PTIP (PAX transcription activation domain interacting protein) is an essential epigenetic regulator in male meiosis. PTIP has been lately identified as part of a histone H3 lysine 4 (H3K4) methyltransferase complex and is essential for embryonic development. H3K4 methylation is an epigenetic mark and is involved in the regulation of genome function through modifying chromatin architecture. Given dynamic chromatin remodeling in spermatogenesis, we recently generated a transgenic mouse model with conditional knockout of PTIP in postnatal male germ cells. Remarkably, our preliminary data reveal that conditional knockout of PTIP in male germ cells leads to spermatogenesis arrest in meiosis prophase I, small testes, azoospermia, and sterility. Accordingly, two Specific Aims will be pursued in this application to test our hypothesis. Specific Aim 1: Further analyze meiotic phenotypes of PTIP- deficient male germ cells. We will accomplish this aim by implementing meiotic chromosome spread and immunofluorescence. Specific Aim 2: Determine PTIP-interacting proteins in spermatocytes. We will employ immunoaffinity chromatography followed by mass spectrometry to identify PTIP's functional partners. Successful completion of this project will considerably further our understanding of the germ cell epigenome during spermatogenesis and broadly define an epigenetic mechanism of cell differentiation and development. Furthermore, collected information may offer insights into unexplained (or idiopathic) infertility in men and may lead to new strategies for male contraception and assisted reproduction.